### What is this PR for?

This PR improve the S3NotebookRepo to adapt the new note file structure. Besides that I use s3proxy to add unit test for S3NotebookRepo. Unfortunately I don't have aws account, so I didn't test it against the real s3. I would be very appreciated if someone can help test it against rest s3.

### What type of PR is it?
[Bug Fix]

### Todos
* [ ] - Task

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-4659

### How should this be tested?
CI pass

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Jeff Zhang <zjffdu@apache.org>

Closes #3675 from zjffdu/ZEPPELIN-4659 and squashes the following commits:

c2b28d14e [Jeff Zhang] [ZEPPELIN-4659]. S3NotebookRepo doesn't work for new note file structure